Minimization of Drive Tests (MDT)
Minimization of Drive Tests (MDT) is a feature in Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) and Universal Terrestrial Radio Access (UTRA). According to MDT, a User Equipment device (UE) is configured to log one or more radio measurement results along with associated information, which may comprise location where the measurement is performed, timing information such as timestamp when measurement is performed, etc. The UE can be configured to log such measurements in low activity state (e.g. idle state) and/or in Radio Resource Control (RRC) connected state. The UE may report the logged information to the network node when the UE goes into the RRC connected state. The network node uses the MDT related information received from the plurality of UEs for network deployment related functions e.g. determination of coverage holes, network planning, configuration of system parameters, etc. The MDT therefore avoids the need for doing drive tests for network planning and optimization.
Figure (FIG. 1 shows an example MDT architecture.
The network node (e.g. enhanced or evolved Node B [eNB]) can also configure the UE to log measurements performed on Multimedia Broadcast Multicast Service (MBMS)-related signals: Multicast Broadcast Single Frequency Network (MBSFN) Reference Signal Received Power (RSRP) (on MBSFN reference signals), MBSFN Reference Signal Received Quality (RSRQ) (on MBSFN reference signals), and Multicast Channel (MCH) Block Error Rate (BLER) (on MCH)—all these measurements can only be configured and performed in MBSFN subframes with MCH transmitted. The measurements can be configured per MBSFN area (up to 8 MBSFN areas) and per MCH (up to 15 MCHs within an MBSFN area).
The requested measurements may be further limited to one or more Public Land Mobile Networks (PLMNs) and/or one or more cells and/or one or more tracking areas and/or specific carrier frequency. The measurements in the measurement logs are also linked to a time stamp and (optionally) positioning information that are available in the UE. The time stamp can be expressed in absolute or relative values. The relative time stamp is defined as the time elapsed from a reference time to the moment the measurement is logged by a radio node. The relative accuracy of the time stamping (aka relative time stamp accuracy) is the drift of the time stamping e.g. ±1 second. It may also be expressed in terms of parts per million (ppm) or parts per billion (ppb) over certain time duration e.g. ±200 ppb over 1 hour. This in turn corresponds to ±0.72 second of drift in time stamp over a period of 1 hour.
Among others, some of the logging configuration parameters include the following, e.g., those received in LoggedMeasurementConfiguration:                Trace reference, trace session and Trace Collection Entity (tce) information        Logging interval        Logging duration        Time information (absolute time in the current cell)        PLMN list        Area list        Target MBSFN area list.Discontinuous Reception (DRX)        
Power consumption is important for UEs using battery or an external power supply and its importance increases with the continued growth of device populations and more demanding use cases. The importance can be illustrated by the following scenarios, e.g.:                For Machine-to-Machine (M2M) use cases like sensors that run on battery it is a major cost to on site exchange (or charge) the batteries for a large amount of devices and the battery lifetime may even determine the device's lifetime if it is not foreseen to charge or replace the battery;        Even for scenarios where UEs may consume power from an external power supply, it may be desirable to consume less power for energy efficiency purposes.        
Enhancing DRX operation is a way to improve battery saving in the UE. DRX makes the UE reachable during pre-defined occasions without resulting in unnecessary signaling. As currently defined, DRX cycles in LTE can at most be 2.56 seconds and thus would not allow for sufficient power savings for UEs that only need to wake-up infrequently (e.g. every few or tens of minutes) for data. Hence, DRX cycle extension is required in order to enable significant battery savings for such UEs. Furthermore, the DRX cycle can be set depending on the data delay tolerance and power saving requirements, thus providing a flexible solution for achieving significant UE battery savings.
Currently, 3GPP is defining enhanced or evolved DRX (eDRX) operation for UEs in CONNECTED mode in LTE and for UEs in IDLE mode in LTE and UTRA. In LTE, the eDRX in IDLE is based on the Hyper System Frame Number (H-SFN) concept.
DRX Configuration for Non-eDRX (Legacy) UE
In LTE, DRX has been introduced as a prominent solution to conserve battery power in mobile terminal. DRX is characterized by the following:                Per UE mechanism (as opposed to per radio bearer);        May be used in RRC_IDLE and RRC_CONNECTED; In RRC_CONNECTED, eNB/UE may initiate the DRX mode when there are no outstanding/new packets to be transmitted/received; in RRC_IDLE                    Second Generation (2G) and Third Generation (3G) terminal use discontinuous reception in idle state to increase battery life time. High Speed Packet Access (HSPA) and LTE have introduced DRX also for connected state                        Available DRX values are controlled by the network and start from non-DRX up to x seconds.        Hybrid Automatic Repeat Request (HARQ) operation related to data transmission is independent of DRX operation and the UE wakes up to read the Physical Downlink Control Channel (PDCCH) for possible retransmissions and/or ACK/NAK signaling regardless of DRX. In the downlink, a timer is used to limit the time the UE stays awake awaiting for a retransmission;        When DRX is configured, the UE may be further configured with an “on-duration” timer during which time the UE monitors the PDCCHs for possible allocations;        When DRX is configured, periodic Channel Quality Indication (CQI) reports can only be sent by the UE during the “active-time.” RRC can further restrict periodic CQI reports so that they are only sent during the on-duration;        eNB does not transmit packets to UE during the sleep mode.For UE in RRC_IDLE        
A UE is set to DRX in idle mode after a prolonged time of air interface inactivity. It's also known as paging DRX, i.e. the time the mobile device can go to sleep between two paging messages which could contain a command for the mobile to wake up again and change back to RRC_CONNECTED state.
FIG. 2 illustrates a procedure for determining an IDLE_DRX cycle in a legacy system.
Referring to FIG. 2, the eNB broadcasts a default DRX value via System Information Block 1 (SIB1). UE can provide a UE specific DRX value to Mobility Management Entity (MME) if UE wants to have a shorter DRX than the default value. Upon triggering a paging for the UE, MME sends the UE specific DRX value together with the paging. Both UE and eNB know the default DRX value and the UE specific DRX value, and the final DRX cycle T is set to the shortest of them. If UE does not provide the UE specific DRX value, the final DRX cycle T is the default DRX value.
One Paging Frame (PF) is one Radio Frame, which may contain one or multiple Paging Occasion(s). When DRX is used, the UE needs only to monitor one Paging Occasion (PO) per DRX cycle.
For UE in RRC_CONNECTED
The following definitions apply to DRX in evolved or enhanced Universal Terrestrial Radio Access Network (E-UTRAN):                on-duration: duration in downlink subframes that the UE waits for, after waking up from DRX, to receive PDCCHs. If the UE successfully decodes a PDCCH, the UE stays awake and starts the inactivity timer;        inactivity-timer: duration in downlink subframes that the UE waits to successfully decode a PDCCH, from the last successful decoding of a PDCCH, failing which it re-enters DRX. The UE shall restart the inactivity timer following a single successful decoding of a PDCCH for a first transmission only (i.e. not for retransmissions).        active-time: total duration that the UE is awake. This includes the “on-duration” of the DRX cycle, the time UE is performing continuous reception while the inactivity timer has not expired and the time UE is performing continuous reception while waiting for a downlink retransmission after one HARQ Round Trip Time (RTT). Based on the above the minimum active time is of length equal to on-duration, and the maximum is undefined (infinite).        
Of the above parameters the on-duration and inactivity-timer are of fixed lengths, while the active-time is of varying lengths based on scheduling decision and UE decoding success. Only on-duration and inactivity-timer duration are signaled to the UE by the eNB:                There is only one DRX configuration applied in the UE at any time;        UE shall apply an on-duration on wake-up from DRX sleep.        
FIG. 3 illustrates an example of DRX mode in LTE. DRX is triggered by means of an inactivity time known as DRX. As illustrated by FIG. 3, the UE activity time may be extended if PDCCH is received during ON Duration time. However, it may also be shortened by a Medium Access Control (MAC) DRX command, upon reception of which the UE stops onDurationTimer and drx-InactivityTimer.
If PDCCH has not been successfully decoded during the on-duration, the UE shall follow the DRX configuration (i.e. the UE can enter DRX sleep if allowed by the DRX configuration):                This applies also for the sub-frames where the UE has been allocated predefined resources.        If it successfully decodes a PDCCH for a first transmission, the UE shall stay awake and start the inactivity timer (even if a PDCCH is successfully decoded in the sub-frames where the UE has also been allocated predefined resources) until a MAC control message tells the UE to re-enter DRX, or until the inactivity timer expires. In both cases, the DRX cycle that the UE follows after re-entering DRX is given by the following rules:                    If a short DRX cycle is configured, the UE first follows the short DRX cycle and after a longer period of inactivity the UE follows the long DRX cycle; if short DRX cycle is used, the long cycle will be a multiple of the short cycle;                            Durations for long and short DRX are configured by the RRC. The transition between the short and long DRX cycles is determined by the eNB MAC commands (if the command is received and short DRX is configured, the UE will (re)start drxShortCycleTimer and use the Short DRX Cycle; otherwise long DRX will be used) or by the UE based on an activity timer                                    Else the UE follows the long DRX cycle directly.eDRX configuration in LTEFor UE in RRC_IDLE                        
Similar to DRX, eDRX configuration parameters are “negotiated” between UE and the network via Non-Access Stratum (NAS). The UE may include eDRX parameters in ATTACH REQUEST or TRACKING AREA UPDATE REQUEST messages. The network shall include eDRX parameters (eDRX cycle; inclusion of paging window length is “For Future Study” [FFS]) in ATTACH ACCEPT or TRACKING AREA UPDATE ACCEPT messages if the UE included eDRX parameters (eDRX cycle) in the corresponding REQUEST message and the network supports and accepts the use of eDRX. The eDRX parameters: eDRX cycle length and Paging Time Window (PTW) length.
UE is configured with a PTW assigned by MME and provided to UE via NAS (3GPP TS 24.301). PTW is characterized by:                Paging H-SFN (PH) (calculated by a formula):                    H-SFN mod TeDRX=(UE_ID mod TeDRX)            UE_ID: IMSI mod 1024            TeDRX: eDRX cycle of the UE, (TeDRX=1, 2, . . . , 256 in hyper-frames) and configured by upper layers                        PTW start (calculated within PH):                    The start of PTW is uniformly distributed across 4 paging starting points within the PH.            PW_start denotes the first radio frame of the PH that is part the paging window and has System Frame Number (SFN) satisfying the following equation:SFN=256*ieDRX, where ieDRX=floor(UE_ID/TeDRX,H) mod 4            PW_end is the last radio frame of the PW and has SFN satisfying the following equation:SFN=(PW_start+L*100−1) mod 1024, where            L=Paging Window length (in seconds) configured by upper layers                        PTW length (configured by higher layers).        
Within a PTW, the UE is further configured with a legacy DRX (FIG. 4).
H-SFN is defined as a new frame structure on top of the legacy SFN structure, where each H-SFN value corresponds to a cycle of 1024 legacy frames and one H-SFN cycle contains 1024 hyper-SFNs (10 bits)—see FIG. 3. All MMEs and eNBs have the same H-SFN, and cells broadcast their H-SFN via SIB1 or SIB1bis.
The PTW is assigned by MME and provided to UE via NAS during attach/tracking area update. The beginning of PTW is calculated by a pre-defined formula.
For UE in RRC_CONNECTED
eDRX procedure for RRC_CONNECTED UE is the same as the legacy case, except that two new DRX cycles have been added: 5.12, 10.28 seconds.
eDRX Capabilities
For LTE, the following eDRX-related capabilities have been agreed in 3GPP:                Cell eDRX capability to support eDRX in RRC_IDLE (eDRXAllowed)        UE eDRX capability to support eDRX in RRC_CONNECTEDCell eDRX Capability (IDLE)        
The cell eDRX capability is broadcasted via SIB1, which is specified in 36.331, v13.0.0:
 SystemInformationBlockType1-v13xy-IEs ::= SEQUENCE { hyperSFN     BIT STRING (SIZE 10))   OPTIONAL, eDRXAllowed     ENUMERATED {true} OPTIONAL, nonCriticalExtension      SEQUENCE { }    OPTIONAL} hyperSFN Indicates hyper SFN which increments by one when the SFN wraps around. eDRXAllowed Indicates if idle mode extended DRX is allowed in the cell. The UE shall not operate in idle mode extended DRX unless eDRXAllowed is set to TRUE.UE eDRX Capability (CONNECTED)
The UE can indicate to e Node B its capability to support eDRX in RRC_CONNECTED via UE-EUTRA-Capability information element in RRC.
 UE-EUTRA-Capability-v13xy-IEs ::= SEQUENCE {  mac-Parameters-r13        MAC-Parameters-r13  OPTIONAL,  nonCriticalExtension       SEQUENCE { }  OPTIONAL}MAC-Parameters-r13 ::=       SEQUENCE {  extendedLongDRX-r13      ENUMERATED {supported}  OPTIONAL} extendedLongDRX Indicates whether the UE supports extended long DRX cycles of 5.12s and 10.24s in connected mode.eDRX Configuration in UTRA
FIG. 6 illustrates eDRX in UTRA. In UTRA, eDRX have been specified only for IDLE state. In eDRX for UTRA, the DRX cycle is prolonged to some seconds which is much longer than the legacy DRX cycles. The DRX cycle consists of a long sleep period, then the UE wakes up to a Paging Transmission Window where there are N_PTW paging occasions with the legacy PS DRX cycle.